Triaxial Shear Strength Tests Research Articles

Study on the Shear Strength and Erosion Resistance of Sand Solidified by Enzyme-Induced Calcium Carbonate Precipitation (EICP)

Sand solidification of earth-rock dams is the key to flood discharge capacity and collapse prevention of earth-rock dams. It is urgent to find an economical, environmentally friendly, and durable sand solidification technology. However, the traditional grouting reinforcement method has some problems, such as high costs, complex operations, and environmental pollution. Enzyme-induced calcium carbonate precipitation (EICP) is an anti-seepage reinforcement technology emerging in recent years with the characteristics of economy, environmental protection, and durability. The erosion resistance and shear strength of earth-rock dams solidified by EICP need further verification. In this paper, EICP-solidified standard sand is taken as the research object, and EICP-cemented standard sand is carried out by a consolidated undrained triaxial test. A two-stage pouring method is adopted to pour samples, and the effects of dry density, cementation times, standing time, and confining pressure on the shear strength of cemented standard sand are emphatically analyzed. The relationship between cohesion, internal friction angle, and CaCO3 formation was analyzed. After the optimal curing conditions are obtained through the triaxial shear strength test, the erosion resistance model test is carried out. The effects of erosion angle, erosion flow rate, and erosion time on the erosion resistance of EICP-solidified sand were analyzed through an erosion model test. The results of triaxial tests show that the standard sand solidified by EICP exhibits strain softening, and the peak strength increases with the increase in initial dry density, cementation times, standing time, and confining pressure. When the content of CaCO3 increases from 2.84 g to 12.61 g, the cohesive force and internal friction angle change to 23.13 times and 1.18 times, and the determination coefficients reach 0.93 and 0.94, respectively. Erosion model test results indicate that the EICP-solidified sand dam has good erosion resistance. As the increase in erosion angle, erosion flow rate, and erosion time, the breach of solidified samples gradually becomes larger. Due to the deep solidification of sand by EICP, the development of breaches is relatively slow. Under different erosion conditions, the solidified samples did not collapse and the dam broke. The research results have important reference value and scientific significance for the practice of sand consolidation engineering in earth-rock dams.

Model of shear strength of ultra-deep fractured sandstone considering fracture morphology

Ultra-Deep fractured tight sandstone gas reservoir, the hotpot unconventional petroleum resources, has been the subject of investigation recently. Due to the presence of natural fractures, drilling in ultra-deep fractured tight sandstone may cause complicated wellbore instabilities, which resulted in a substantial challenge to the efficient exploitation and production of tight gas resources. In order to understand the rock mechanical properties of the ultra-deep fractured tight sandstone under shear stress, tight sandstone core samples were artificially shear-fractured, the 3D characteristics of the fractures were reconstructed by CT scanning, and the fracture aperture and roughness characteristics were systematically evaluated. Simultaneously, triaxial shear strength tests were conducted to evaluate the mechanical properties of fractured rock subjected to varying confining pressures. Considering the contact relationship of the fracture surface during shearing, two indexes of average effective shear angle and contact area ratio were proposed to correct the fracture roughness, and a new fracture shear strength model was constructed, which served as the failure criterion for the fractured formation. The results show that the intact rock has a high compressive strength and can only collapse under conditions of substantial differential stress. Consequently, shear failure of fracture ought to be the primary source of wellbore instability. The fracture aperture's spatial distribution is irregular and conforms to a Gaussian distribution. A clear anisotropy in the fracture roughness is present, and there is less roughness along the initial shear failure path and more roughness perpendicularly. With an increase in normal stress, the fracture's peak shear strength rises, while the friction angle steadily falls. The innovative model's accuracy in predicting shear strength is higher than that of the weak-surface model and the JRC-JCS model.

Incorporation of plasticity and temperature effects on bituminous stabilised materials in a multi-layered pavement model

The mechanical properties of Bituminous Stabilized Materials (BSMs) have been widely studied in the past, however the influence of temperature on their non-linear constitutive properties has never been addressed. In this research, the role of temperature on the mechanical properties of a BSM prepared with asphalt emulsion and hydrated lime was investigated. Triaxial shear strength tests were performed at different temperatures and confining pressures. The experimental results allowed the development of finite element models defining the local stress and temperature-dependent mechanical properties. An approach for incorporating plasticity and temperature effects through a 2D axisymmetric finite element model of multilayer pavement was developed. Three pavement structures were subjected to the same load applications at different temperatures. Their responses were compared to determine the most critical scenario. In conclusion, plasticity and temperature are key factors for BSMs analysis and design while linear-elastic assumptions lead to overestimating the service life of rehabilitated pavements.

Plasticity-Based Method for the Design and Analysis of Cold Recycled Pavement Layers

Asphalt cold recycling technologies allow the use of up to 100 % reclaimed asphalt pavement (RAP) from an existing asphalt pavement to construct new base layers and new subbase layers. Bituminous stabilized materials (BSM), obtained from cold recycling technologies, are susceptible to traffic-induced accumulation of permanent deformation. In the current design methods, BSM pavement layers are typically assumed to have a linear elastic response under traffic loading application, and this does not account for plasticity initiation and accumulation in the material. This research study provides insight on how to integrate plasticity behavior in numerical simulations of rehabilitated pavement structures with BSM. A BSM mixture prepared with 1.5 % cement by weight of total mixture and 1.5 % foamed asphalt binder content by weight of dry aggregates is characterized in the laboratory. Subsequently, a three-dimensional elastic-perfectly plastic finite element model is developed for the simulation of triaxial shear strength tests. Lab measured force-displacement curves are matched with the numerical simulations at different confining pressures. The calibrated mechanical properties are then used as input parameters in multilayer pavement models for pavement evaluation. Different pavement structures with and without BSM layers are compared in terms of a maximum allowable number of loading repetitions before reaching a 20-mm rut depth on the surface of the pavement. The results indicate that structures with BSM and traditional granular base layers yield comparable results in terms of rutting potential. In addition, the study presents a framework for the design and analysis of cold recycled pavement layers and a method to integrate the plasticity behavior of partially-bonded and unbounded layers for pavement evaluation.

Study on Soil Water and Suction Stress Characteristics for Unsaturated Clay Soil of Airport Engineering Based on Laboratory Tests

For the unsaturated soil in Feidong China, this study examined the suction stress characteristics based on the soil-water characteristic curve (SWCC), which was different from traditional research ideas. At the same time, the unsaturated consolidation device was adopted for SWCC tests, with consideration of the influence of yielding stress of soil, which was different from the traditional test approach of the soil-water characteristic curve. The results were estimated using the van Genuchten model, which was revealed that this is well-fit for the studied unsaturated soil, and the triaxial shear-strength tests were conducted with suction control. Then, the suction stress characteristic curve (SSCC) was analyzed, and SWCC-predicted data were compared with triaxial test-derived suction stress data. For the studied unsaturated soil, the deviatoric stress increased with the net inner stress p − u a at the same matric suction. At the same net inner pressure, the deviatoric stress increased with the matric suction, which verified the hardening activity of matric suction on the tested unsaturated soil strength. Besides, triaxial test-derived suction stress data greatly conformed to SWCC data-derived SSCC that was determined using identical parameters used in the SWCC model.

Effect of Groundnut Shell Ash on Laterite Soils Stabilized with Lime for Civil Structures

This study considered the practicality of groundnut shell ash (GSA) on laterite stabilized with lime for civil structures. Three site locations of lateritic soil named specimen I, II and III were assembled from Ifewara, Atakunmosa West Local Government Area, Ilesa East Local Government Area, and Ilesa West Local Government, all situated in Osun State, Nigeria. Preliminary tests were wrapped up on the soil specimens in their characteristic states and when stabilized with optimum lime. Compaction, California Bearing Ratio (CBR) and undrained triaxial shear strength tests were performed when fluctuating paces of 2 %, 4 %, 6 %, and 8 % of GSA were included to the soil specimens at optimum lime. The Atterberg limits tests showed a critical decrease in plasticity index for all the soil specimens when stabilized with lime. Compaction test showed a lessening in the maximum dry density from 1732 kg/m3 to 1651 kg/m3 for specimen I, 1874 kg/m3 to 1621 kg/m3 for specimen II and 1683 kg/m3 to 1655 kg/m3 for specimen III on stabilizing with lime, presentation of GSA to stabilized lime-soil decreases the maximum dry density for all the soil specimen with specimen I diminished to 1642 kg/m3, 1595 kg/m3, 1611 kg/m3 and 1611 kg/m3 at 2 %, 4 %, 6 % and 8 % GSA substances individually. Addition of GSA substances enhanced the engineering properties of laterite stabilized with lime as the unsoaked CBR values expanded for all the soil specimens. At optimum lime measurements, addition of 4 % GSA expanded the shear strength to 110.74 kN/m2 and 127.53 kN/m2 for specimens I and II individually while at 6 % GSA addition, the shear strength of specimen III was peak 118.24 kN/m2. The expansion in shear strength further affirms the improvement prior shown in the geotechnical properties of lateritic soil with the addition of groundnut shell ash. addition of 2 % GSA content extended the triaxial shear strength from 60.43kN/m2 to 188.36kN/m2 for specimen I, and at 4% GSA content, both soil specimens II and III expanded from 19.19kN/m2 to 201.48kN/m2 and 30.62kN/m2 to 111.65kN/m2 separately. Conclusively, GSA improved the durability and strength of lateritic soils stabilized with lime for civil structures.

Study on shear behavior of kaolinite contaminated by heavy metal Cu (II).

Numerous studies have shown that the invasion of the chemicals plays an important role on the geomechanical properties of the soil. This article aims to investigate the shear behavior of contaminated soil by laboratory tests and develop an extended shear strength model on the basis of the experimental results. In order to explicitly evaluate the effect of solution concentration on the shear strength behavior of soil, the remolded samples of kaolinite mixed with different concentrations of CuCl2 solutions were prepared to carry out a series of consolidated-undrained triaxial shear strength tests. The results indicate that different CuCl2 solution concentrations have significant influence on the shear strength property of kaolinite. With the increase of CuCl2 solution concentration, the shear strength of soil displays a declining tendency, and the strength properties including cohesion and internal friction angle are also reduced, which indicates the Cu (II) that existed in the soil samples has deteriorated the soil strength strongly. Based on the experimental results, an extended Mohr-Coulomb strength model for contaminated soils has been proposed by introducing osmotic suction as a macro variable parameter. The conclusions in this study can provide reference for pollution prevention of existing and future foundations.

Railroad ballast behavior in triaxial compression tests with and without geogrid

This paper highlights recent laboratory studies at Emperor Alexander I St. Petersburg State Transport University, using large-scale cylindrical triaxial apparatus, which helped quantify the geogrid performance in railway ballast. The triaxial shear strength tests were conducted on ballast with and without geogrid. Effect of geogrid layers quantity on the ballast stress-strain behaviour were studied. The experimental results indicate that the geogrids have improved ballast bearing capacity up to 25%.

Hydromechanical responses of coal powders by CO2 adsorption

Enhanced coal bed methane recovery is a promising method to produce methane gas while storing carbon dioxide (CO2) in coal bed seams. Although the adsorption of the injected carbon dioxide on coals is known to cause swelling and softening of coals, its roles in the worsening of transport and mechanical characteristics have not been fully quantified. Thus, this study examined the effect of carbon dioxide adsorption on the mechanical characteristics (stiffness and strength), permeability and P-wave velocity of coals by using a modified triaxial strength apparatus. Anthracite coal powder samples from Kyung-Dong, Korea, were used in this study. It was confirmed that carbon dioxide adsorption reduced the permeability due to the carbon dioxide-induced swelling and increased the P-wave velocity, possibly owing to additional consolidation of softened coals. Upon completion of carbon dioxide adsorption, a series of multistage triaxial shear strength tests were performed under a consolidated-drained condition. Clear reductions in the mechanical strength and stiffness of the coal samples were observed, confirming the carbon dioxide-induced softening effect on coals. The results suggest that carbon dioxide adsorption heavily affects the mechanical and hydraulic properties of coals, in particular, confirming the carbon dioxide-induced swelling and softening effects on coals.

Improvement in engineering properties of subgrade soil due to stabilization and its effect on pavement response

This paper presents laboratory investigation of stabilization of subgrade soil. One type of soil and three types of stabilizers i.e., hydrated lime, class F fly ash and polypropylene fibres are selected in the study. Atterberg limit, compaction, california bearing ratio (CBR), unconfined compressive strength and triaxial shear strength tests are conducted on unstabilized and stabilized soil for varying percentage of stabilizers to analyze the effect of stabilizers on the properties of soil. Vertical compressive strains at the top of unstabilized and stabilized subgrade soil were found out by elasto-plastic finite element analysis using commercial software ANSYS. Strategy for design of optimum pavement section was based on extension in service life (TBR) and reduction in layer thickness (LTR). Extension in service life of stabilized subgrade soil is 6.49, 4.37 and 3.26 times more due to lime, fly ash and fibre stabilization respectively. For a given service life of the pavement, there is considerable reduction in layer thicknesses due to stabilization. It helps in reduction in construction cost of pavement and saving in natural resources as well.

Research on Large-Scale Tri-Axial Shear Strength Tests in Slide Zone Soil of Huangtupo Landslide

Research on Large-Scale Tri-Axial Shear Strength Tests in Slide Zone Soil of Huangtupo Landslide

Characterization of geogrid reinforced ballast behavior at different levels of degradation through triaxial shear strength test and discrete element modeling

Recent research efforts at the University of Illinois have aimed at studying geogrid applications in railroad track structures, specifically focusing on ballast and subballast reinforcement. Ballast, typically comprising large sized aggregate particles with uniform gradation, is an essential layer in the railroad track substructure to facilitate load distribution and drainage. The primary mechanism of load transfer within the ballast layer involves inter-particle contact between ballast particles. Similarly, the effectiveness of ballast reinforcement with geogrids is primarily governed by the geogrid-aggregate interlock. Such interaction and the effectiveness thereof can change significantly as the level of grain size and shape degradation or fouling increases in the ballast layer with accumulation of train traffic. Although several studies in the past have investigated the effects of geogrid reinforcement on ballast shear strength and permanent deformation behavior, the effectiveness of geogrid reinforcement at different levels of ballast degradation needs to be further understood. In this study, monotonic triaxial shear strength tests were conducted on both new and degraded ballast materials with and without geogrid reinforcement. Two geogrid types, with square- and triangular-shaped apertures, were used in the laboratory to calibrate an aggregate imaging-based Discrete Element Method (DEM) modeling approach, which is capable of creating actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. The DEM model was observed to adequately capture the shear strength behavior of geogrid-reinforced triaxial ballast specimens prepared using both new and degraded ballast samples.

Characteristics of structural loess strength and preliminary framework for joint strength formula

The strength of structural loess consists of the shear strength and tensile strength. In this study, the stress path, the failure envelope of principal stress (Kf line), and the strength failure envelope of structurally intact loess and remolded loess were analyzed through three kinds of tests: the tensile strength test, the uniaxial compressive strength test, and the conventional triaxial shear strength test. Then, in order to describe the tensile strength and shear strength of structural loess comprehensively and reasonably, a joint strength formula for structural loess was established. This formula comprehensively considers tensile and shear properties. Studies have shown that the tensile strength exhibits a decreasing trend with increasing water content. When the water content is constant, the tensile strength of the structurally intact soil is greater than that of remolded soil. In the studies, no loss of the originally cured cohesion in the structurally intact soil samples was observed, given that the soil samples did not experience loading disturbance during the uniaxial compressive strength test, meaning there is a high initial structural strength. The results of the conventional triaxial shear strength test show that the water content is correlated with the strength of the structural loess. When the water content is low, the structural properties are strong, and when the water content is high, the structural properties are weak, which means that the water content and the ambient pressure have significant effects on the stress-strain relationship of structural loess. The established joint strength formula of structural loess effectively avoids overestimating the role of soil tensile strength in the traditional theory of Mohr-Coulomb strength.

Mechanical Characterization of Superpave Gradation between Passing through and below Restriction Zone

Mechanical characterization of the Superpave gradations passing through and below the restriction zone is presented through a series of strength tests, which include semi-circular bending test, dynamic modulus test, indirect tensile creep compliance test, indirect tensile fracture energy test, and triaxial shear strength test. The investigation is firstly to get these two different aggregate structures using Superpave Level 1 mix design method. The optimum asphalt content for the gradation mixtures passing through and below the restriction zone are 5.0% and 4.7%, respectively. The test specimens which were blended at the optimum asphalt content for each gradation mixture were prepared and compacted at four percent of void ratio using Superpave Gyratory Compactor. The Marshall mix design property results indicate that the higher stability values, indirect tensile strength and flow values are observed for the gradation mixtures passing through the restricted zone (TRZ gradation). It appears that these mixtures might possess better resistance against fatigue cracking and less resistance against rutting compared to the gradation mixtures passing below the restricted zone (BRZ gradation). Moreover, the TRZ gradations exhibit the higher fracture toughness in the semi-circular bending tests, the lower exponential values in the creep compliance tests, and the higher fracture energy in the indirect tensile strength tests. All results indicate that the TRZ gradations could have a better capability against fatigue cracking. However, the BRZ gradations reveal the higher dynamic modulus observed at longer reduced time in the dynamic modulus master curves, the more reduced time required to reach the same strain in the repeated loading triaxial tests, and the slightly larger value of φ obtained in the triaxial shear tests, all illustrate their superior performance against rutting. Based upon these experimental results, it can be stated that the TRZ gradations exhibit better resistance against fatigue cracking, whereas, the BRZ gradations possess greater performance in rutting resistance.

Relationship between the Soil-Water Characteristic Curve and the Suction Stress Characteristic Curve: Experimental Evidence from Residual Soils

The part of effective stress resulting from soil moisture or soil suction variation can be defined by the suction stress characteristic curve (SSCC). For a given soil, the SSCC can be experimentally determined from shear-strength tests. Recent work shows that the SSCC can be uniquely linked to the soil-water characteristic curve (SWCC). The uniqueness of the SSCC determined from both shear strength and soil moisture retention tests is examined for several residual soils in Korea. The validity of the effective stress principle is demonstrated by showing that effective stress-based on the SSCC describes the same unique failure criterion as that for the saturated failure criterion. The measured SSCCs are also shown to predict the soil-water retention curves within a few percentage. The SWCCs of these residual soils, determined directly from soil moisture retention tests, also accord well with the SSCCs determined directly from triaxial shear-strength tests with the difference within several tens of kPa. Therefore, we show that the suction stress characteristic curve or soil-water retention curve alone can be used to describe both the effective stress and soil-water retention characteristics of variably saturated soils. DOI: 10.1061/ (ASCE)GT.1943-5606.0000564. © 2012 American Society of Civil Engineers.

Guide on the Selection of Appropriate Laboratory Stress Levels for the Flow Number Test

This study examines the use of the triaxial shear strength parameters (c and φ) in providing guidelines for selecting the appropriate laboratory stress level for the flow number test of asphalt mixtures. The approach is based on the concept that the flow number (tertiary flow) is inversely related to the applied stress-to-strength ratio of the material. The correlation of the flow number with the stress-to-strength ratio can provide a good estimate of the stress level that will yield tertiary flow within a reasonable testing time. It was also realized that the triaxial shear strength test is not routinely conducted by Department of Transportation agencies and testing laboratories. Therefore, a database of triaxial shear strength test data was used to develop predictive models of the shear strength parameters for a particular asphalt mixture. The models’ predictor variables were based on the volumetric properties of 46 different asphalt mixtures and a total of 276 test results. Regression models to estimate the c and friction (φ) parameters had good statistical measures of model accuracy. The models were used to develop guidelines for laboratory stress-to-strength ratios to achieve tertiary flow within a reasonable testing period.

Geotechnical characterization of calcareous sediments from the Dry Tortugas Keys and Marquesas Keys CBBL SRP study sites, Lower Florida Keys

Geotechnical characteristics of carbonate sediments from two test sites (Dry Tortugas Keys and Marquesas Keys) in the Lower Florida Keys were investigated as part of the Coastal Benthic Boundary Layer Special Research Program, through an extensive field coring and laboratory testing program conducted by the Marine Geomechanics Laboratory of the University of Rhode Island. Based on results from physical measurements, water content and wet bulk density values for both sites generally showed large variations in the upper 25 cm and little variation below this depth. Sediment samples exhibited low plasticity or nonplastic characteristics. Constant-rate-of-deformation consolidation test results showed strong apparent overconsolidation (stress state ratio >7.5) in the surface sediments (upper 50 cm) at the Dry Tortugas Keys, and light overconsolidation (stress state ratio <1.5) below 50-cm depth at the Marquesas Keys site. In-situ permeability values were between 10–4 and 10–7 cm/s at both sites and showed no strong depth dependence in the upper 2 m. Undrained shear strength profiles for Dry Tortugas Keys sediments indicated a marked stiffening with depth, whereas the Marquesas Keys sediments showed a gradual increase with depth. Consolidated isotropically undrained triaxial shear strength test results indicate that the undisturbed sediments had an average effective angle of internal friction of 38°, which is not fully realized until large axial strains on the order of 11% have accumulated. Evidence of cementation was not found in triaxial compression or consolidation test results. The general behavior and characteristics of these sediments are similar to those of granular materials, which is primarily due to their high calcium carbonate contents and lack of cementation.

Geotechnical Properties of Paper Mill Sludges for Use in Landfill Covers

This study investigates the geotechnical properties of seven paper mill sludges. Paper mill sludges have a high water content and a high degree of compressibility and behave like a highly organic soil. Consolidation tests reveal a large reduction in void ratio and high strain values that are expected due to the high compressibility. Triaxial shear-strength tests conducted on remolded and undisturbed samples showed variations in the strength parameters resulting from the differences in sludge composition (i.e., water content and organic content). Laboratory permeability tests conducted on in-situ specimens either met the regulatory requirement for the permeability of a landfill cover or were very close. With time, consolidation and dewatering of the paper sludge improved the permeability of cover. Freezing and thawing cycles increased the sludge permeability about one to two orders of magnitude. Maximum permeability changes occurred within 10 freeze and thaw cycles.